The present invention relates to a novel preserving agent for various food products, especially but not exclusively useful for the preservation of fish, and which is prepared by extraction from tea leaves. It also provides a method for preparing the preserving agent and food compositions, especially sea food compositions, containing this agent as a preservative.
Sea food products, such as fish, various types of processed fish, crustaceans and the like, are renown for having a limited shelf life and especially a limited period of optimum freshness. There are several reasons for this. Typical causes of inferior quality of fishery products include putrefaction and decomposition of fishery products as a result of the propagation of microorganisms and the oxidation of lipid components in the fishery products.
Many means, such as drying, salting, smoking, heating, freezing or refrigerating, controlled atmosphere storage, radiation sterilization, vacuum packaging, gas packaging, deoxidizing agent sealed packaging, and addition of biocides or preserving agents, have been proposed in order to inhibit the propagation of microorganisms. Many other methods, such as freezing, refrigerating, vacuum packaging, gas packaging, deoxidizing agent sealed packaging, and addition of antioxidants, have been proposed in order to prevent oxidation of lipid components. The oxidation of lipid components leads to alterations in flavor and taste; this is generally accompanied by other undesirable degenerative changes, such as changing color, decomposition of vitamins and degeneration of proteins, and so the prevention of lipid oxidation is very important in order to stabilize the quality of fishery products and to maintain their nutritive value. There are many means to prevent the oxidation of lipids, typified by the above examples, but the most easy and effective means is to use an antioxidant. To date, many types of antioxidant have been used, but it is impossible to escape the conclusion that the action of these known antioxidants has, in general, been insufficient because of poor effectiveness, low durability of effects, and adverse influences on color and flavor.
Tea leaves are known to include active components having some form of preservative activity as a result of their antioxidant properties, and so some attempts have been made to use tea leaf components as preservatives, although not specifically for sea foods. However, their use on a large scale has not so far been possible. To date, water, alcohol, aqueous alcohol, acetone, and aqueous acetone have been used in an attempt to extract tea leaves, however, every method of extraction previously tried has been found to give rise to a variety of problems. For example, the amount of active preservative matter extracted may be too low, the amount of energy required for concentration may be too uneconomical, removal of coloring matter by me difficult, and safety of the workforce may be put at risk by certain extracting solvents. Accordingly, commercial utilization of these prior processes has been restricted. When obtained by extraction with water, the extracts had reduced activity, because only a relatively small quantity of active ingredients were extracted and because the quantity of polysaccharides and proteins in the extracts was high. When obtained by extraction with organic solvents (such as alcohols, e.g. ethanol, aqueous alcohols, acetone or aqueous acetone), the concentration of active ingredients was high, however, large quantities of coloring matter and lipids were simultaneously extracted, and so a further removal step to eliminate these unwanted materials was necessary; this gave rise to many additional problems, such as the loss of recycled organic solvents and the danger of the solvents catching fire.
We have now found that an acidic aqueous medium containing polyhydroxy compounds is capable of extracting active components effectively without contamination by ineffective coloring matter, and have also found that the solution obtained after the extraction could itself be used as quality preserving agent (i.e. a preserving agent which not only preserves the food but also maintains the quality as though the food had only just been processed).
Moreover, we have also discovered that the combination of the acidic medium and the polyhydroxy compound together with a tea extract (however prepared) also provides a quality preserving agent, especially for sea foods.